Arch-rival Maria Sharapova is again expected to be the American's main challenger, with injuries marring the preparation of other leading hopefuls including Simona Halep, Caroline Wozniacki and Victoria Azarenka.

Melbourne Park has been a happy hunting ground for Williams, who claimed five of her 18 career majors at the venue - the first way back in 2003.

But the 33-year-old's most recent success Down Under was in 2010 and her erratic form in this month's Hopman Cup raised fears she faces a failure similar to last year, when she crashed out in the fourth round.

Always supremely confident, Williams, who will meet Belgian Alison van Uytvanck in the first round, blamed her early season troubles on fatigue and declared she can play "two thousand times better".

"I feel like I'm getting back into the groove... I'm getting there," she said.

Williams held on to the top ranking for the entire 2014 season, ending the year with a flourish to take the US Open and the WTA Tour Championship.

Another Australian title would take her to clear second on the all-time Grand Slam winners list behind Steffi Graf's 22, having joined Chris Evert and Martina Navratilova on 18 with her win at Flushing Meadows.

World number two Sharapova scoffed at suggestions Williams was past her best.

"You can't say someone is on the edge of the cliff when they are number one in the world," the Russian said. "She still came through and finished the season strong.

"Even though she's 33 years old she's still very strong and very powerful and has so much experience -- that shows when she plays on court."

Sharapova, a winner at Melbourne Park in 2008, finally shrugged of a nagging shoulder injury last year to claim her fifth Grand Slam title at the French Open.

There are no questions about the world number two's form after she posted a sensational come-from-behind win over Ana Ivanovic in the Brisbane International warm-up event, shaking off the cobwebs before the Australian Open.

"I couldn't have asked for better preparation," said the 27-year-old, who could snatch the top ranking off Williams with a second championship this month.

Last year's champion Li Na of China will not defend her title after retiring in September, while losing finalist Dominika Cibulkova's career has stalled since her appearance in the 2014 decider.

Former world number one Azarenka is eyeing a comeback after winning back-to-back Australian titles in 2012 and 2013, only to slump to 41 in the world after a disastrous 2014 during which she struggled with depression and a string of injuries.

"For me, in my mind, I skipped the whole season, I was never healthy, I was never prepared," said the Belarusian, who faces a tough first-round clash against American Sloane Stephens, a semifinalist two years ago.

World number three Halep of Romania is considered the player most likely to join the Grand Slam club, but her Australian lead up has been derailed by a stomach bug picked up in China, leaving her short of match practice.

Wozniacki has also struggled, with an injured wrist, while Serb Ivanovic looms as a dark horse after quietly rising to fifth in the rankings, citing a new-found maturity that has settled the nerves that once dogged her big-match appearances.

"It shows on and off the court that I'm much more relaxed, I'm much more content with myself," said the 26-year-old former world number one, who was a quarterfinalist in Melbourne last year.